A wayward genius and his chambers of horror

The visceral punch of a Francis Bacon painting is beyond dispute. But at Tate Britain's world-class exhibition - which brings together 100 of his works and reconstructs the photo-plastered walls of this London studio - we come face to face with the existential agony at the heart of his anarchic vision

There are exhibitions - rare, superbly curated - that redefine an artist for a generation. The presentation of 100 works by Francis Bacon at Tate Britain until January, then at the Prado and the Metropolitan Museum in New York, is just such a world-class event. Everyone knows what a Bacon looks like, and since his death in 1992 there have been other shows to remind us, but none has revealed quite as clearly as this one just how mysterious and anarchic his art remains.

Take the content of the paintings. What exactly is going on here? Clearly there are the simple auction-house categories: screaming popes and writhing figures, suicides and crucifixions, grand triptychs of agony and violence bought for record sums by Russian oligarchs. And perhaps it seems that violence must therefore be Bacon's theme: bodies splayed and disembowelled, heads twisted and split as in some motorway pile-up. The naked bulb dangles over an amputee (or so it seems). Tobacco bursts from the stubbed fag like the innards of a corpse and the suicide retches his last breath. Even the beds on which lovers grapple are more like mortuary slabs.

It is certainly true that Bacon did not paint flowers - although there is in fact an amazingly eerie image of hydrangeas in the opening room - and that his popes and martyrs are consummate horror shows. Ectoplasm, gore, mucus, all sorts of nameless substances are evoked and even imitated by the paint itself, and his stated ambition was to make the pictures look 'as if a human being had passed between them, leaving a trail of the human presence as a snail leaves its slime'.

But Bacon was a gregarious Soho bohemian. His people are almost always portraits of lovers and friends. What might look like botched surgery - and like is the operative word since one is always searching for analogies to make sense of his art - is performed on images of drinking companions. Nor could anyone fail to notice just how gorgeous and balletic his pictures are, with their jewel-rich colours and precise choreography; or that his draughtsmanship is so buoyant and deft, even cartoon-like.

John Berger long ago compared Bacon to Walt Disney and, before reeling at the supposed heresy, ask yourself whether these paintings don't have a similar sort of fiendish exuberance in their leaping lines and curves. Tom the cat runs smack into the frying pan, his face flattens, but he bounces back. What is shocking is that Bacon's figures stay stuck in their extreme distortions.

The truth is that time passes and Bacon no longer comes across as the master of the bloody chamber, of images of torture and degradation the like of which had never before been seen in British art. This is partly because he is practically an old master by now, sanctified in museums the world over, his newness erased by familiarity, his revelations superseded by the pictures of real-life horror that flood into our living rooms. But it is also because what he made of his subject matter now seems so much more important - and this is the true action of the paintings: Bacon's obsessive reinvention and restatement of those isolated figures in their cages and cells.

A very early work from 1945, for instance, shows a howling woman bent over naked, a man's overcoat slung across her hindquarters. It is a subject fit for any number of 20th-century artists. But what makes it so devastating here is some sort of nerve-wracking tension between the ravishing orange backdrop and the disembodied mouth with its animal teeth, between the beautifully described tweed of the coat and the outlandish body forms, anticipating David Lynch's Eraserhead by 30 years. And all capped by a funereal umbrella: once seen, never forgotten.

It is a tremendous piece of image-coining and there are so many others in this show, the screaming popes immured in their thrones, the stripped child lolloping on all fours, the dog straining at its chain in the drowning darkness. And put like this, the irresistible comparison ought to be Goya. But the stylishness, the sheer operatic charge of these works has nothing to do with the Spaniard, no matter that Bacon studied the old masters from first to last, harking back to them in his heavy gold frames.

Often, Bacon's showmanship is deliberately apparent. The way he uses the rough reverse of the canvas, the colour seeping into the hessian like blood - burnt orange, royal purple, midnight blue, crimson - or congealing stickily on the surface. The way he keeps every flailing figure in check with a precise geometry of glass boxes, elliptical arenas, the vertical striations of those dividing curtains at the back that suggest that this is just the ante-chamber to something worse.

Look at a particularly camp pope in a monocle - one revelation here is of Bacon's humour - wedged to the waist in his chair like Winnie in Happy Days (Bacon, incidentally, precedes Beckett) and you see that what appears to be an accidental black spatter has been primped up with red so that it looks as if the painting itself spurts blood.

But it is what Bacon does to the figures themselves that resists analysis. The curators of this show have reconstructed the photo-plastered walls of his London studio, about which so much nonsense has been written as if Bacon simply transcribed Eisenstein, Eadweard Muybridge, photos of Nuremberg, textbook shots of mouth diseases or patients positioned for X-rays. This proves crucial. It shows that Bacon never paints an exact moment of violence, nor its aftermath, nor anything captured in a photograph; he invents some split-second transition - his characteristic stop-start mutation.

And where do those wildly aberrant faces come from? They might recall Henry Tonks's studies of First World War soldiers, but Bacon is not recording actual injuries; and this is not just some new variant of modernism either. The eyes want to straighten them out, these heads, put them back together. But the mind cannot.

One of the greatest works here is also the smallest, a portrait of Bacon's lover George Dyer. A nearby photograph shows the same handsome profile, the curved nose echoed by the gleaming black quiff. But the painting, with its swerves and swipes, despite being instantly recognisable, is another thing altogether. Photo-real yet caricatural, molten but graphic, muscular and yet diaphanous, it moves seamlessly through its transitions. Whatever Dyer once was before his suicide, he has become a force-field of deathless matter.

No stories, only images: that was Bacon's claim for his art and even though the late works seem to imply a narrative with their props and locations - the hotel, the telephone, a door flung open, a man hunched over a mirror - they never resolve into simple conclusions. His images are indelible, irrational and beyond summary, and his modest ambition for them - that they should be as vividly realised as possible - has surely turned out to be true.

Join Guardian art critic Jonathan Jones on a walking tour through Tate Britain's Francis Bacon retrospective, and discover how the Irish-born painter emerged as one of the greatest artists of the 20th century